The present invention relates to a seamless cylindrical printing blanket, which is suitably used in a high-speed web offset printing press.
A printing blanket of the prior art was in the form of a plate, and was used by winding around a blanket cylinder of a printing press. With this construction, however, there arose a problem that a seam portion is formed in a printing blanket and a pressing force of a plate cylinder varies whenever the seam portion passes through a nip portion between the blanket cylinder and the plate cylinder to cause vibration and shock load, resulting in deterioration of the printing quality. Particularly, deterioration of the printing quality was remarkable at the time of high-speed printing where the blanket cylinder rotates at high speed of not less than 1,100 rpm.
Therefore, a cylindrical printing blanket having no seam in a circumferential direction has recently been suggested as a printing blanket suited for high-speed printing using a web offset printing press and the like. As shown in FIG. 4, such a cylindrical printing blanket 20 comprises a compressive layer 22, which is porous and seamless, a non-expansion layer 23 and a seamless surface printing layer 25 and a cylindrical sleeve 11 to be fit over a blanket cylinder, the layers 22, 23 and 25 being laminated in this order on the outer peripheral surface of the cylindrical sleeve 11 through seamless adhesive layers g1, g2, g3 (see Japanese Patent Laid-Open Publication No. 5-301483), respectively.
Among the above layers, the surface printing layer is made of a volume-non-compressive (that is, non-compressive as a change in volume does not arise even if plastic deformation is applied) elastomer and absorbs ink (not shown) from a printing plate 42 at a nip portion 41 formed between a printing blanket 20 and a plate cylinder 40 (see FIG. 5).
The compressive layer 22 is formed by applying a coating solution containing an elastomer such as rubber, drying the solution and optionally curing (or vulcanizing) in a case where the rubber is employed as the elastomer. The compressive layer 22 is volume-compressive because of its porous structure (that is, the volume is reduced by compression) and, therefore, the vibration absorbing property and pressure absorbing property are imparted to the whole printing blanket 20, thereby to inhibit bulging in the neighborhood of the nip portion 41 and to prevent deformation of an image such as slur and double in the printing direction (i.e. circumferential direction x of blanket) of the blanket 20.
At the time of high-speed printing; however, there arises a problem that a shear deformation is applied to the blanket 20 at the nip portion 41 and, therefore, the image is deformed in the circumferential direction x of the blanket to cause slur and double.
Therefore, for the purpose of inhibiting slur and double, the non-expansion layer 23 is provided on the compressive layer 22. This non-expansion layer 23 is formed by spirally winding a wire 17 such as a thread in the circumferential direction x of the blanket with applying a tension. By providing such a non-expansion layer 23, the shear deformation at the nip portion 41 of the blanket 20 can be reduced and the deformation of the printed image in the circumferential direction x of the blanket can be inhibited.
However, there is a limit in effect of inhibiting the above shear deformation by providing the non-expansion layer 23. For example, in a high-speed printing at not less than 1,100 rpm, when the thickness of a surface printing layer 250 to be provided on the top of the non-expansion layer 23 becomes larger as shown in FIG. 6, it becomes impossible to obtain sufficient pressure absorbing effect due to the compressive layer 22 because the layer 250 is volume-non-compressive and the distance from the surface of the blanket 200 to the compressive layer 22 (i.e. thickness of blanket 200) becomes larger. As a result, when the plate cylinder 40 is pressed onto the blanket 200, bulging 43 (bulge deformation) is formed on the surface printing layer 250.
When the bulge deformation becomes larger, the nip width also becomes larger. Therefore, the solid inking property is improved but the halftone dot becomes thick due to dot gain, resulting in deterioration of the printing reproducibility. When the thickness of the surface printing layer 250 becomes larger, it becomes impossible to obtain the effect of inhibiting the shear deformation of the blanket 200 by the non-expansion layer 23. As a result, remarkable slur and double make it impossible to put to practical use.
On the other hand, when the surface printing layer 25 is thin as shown in FIG. 5, the shear deformation hardly arises and the reproducibility of the shape of the halftone dot is improved. However, since the non-expansion layer 23 is formed by winding a wire with applying a tension thereon, it contacts closely with the peripheral shape of the printing plate 42 and is not deformed and, therefore, the nip width 41 becomes smaller. At the time of high-speed printing at not less than 1,100 rpm, the time of contact between the printing plate 42 and blanket 20 becomes considerably short and transfer-of ink becomes insufficient, resulting in deterioration of the inking property of ink.
In such way, when the thickness of the surface printing layer is decreased, the halftone dot reproducibility is good and problems such as slur and double do not arise but the inking property of ink is deteriorated. On the other hand, when the thickness of the surface printing layer is increased, bulging arises and the nip width increases and, therefore, the inking property is improved but the dot gain arises to deteriorate halftone dot reproducibility. Besides, when the thickness of the surface printing layer is further increased, slur and bulging arise.
Accordingly, in order to practically satisfy both of the inking property of ink and halftone dot reproducibility, it is necessary to strictly adjust the thickness of the surface printing layer (specifically, the thickness of the surface printing layer is limited to about 0.4 mm to satisfy both the inking property and halftone dot reproducibility) and a scatter in quality of the product is liable to arise. At the time of high-speed printing at not less than 1,100 rpm, it becomes further difficult to satisfy both the inking property and halftone dot reproducibility since it is required to further reduce the thickness of the surface printing layer.
On the other hand, as shown in FIG. 7, in a case where a printing blanket 30 is provided with a compressive layer 32 only between a non-expansion layer 33 and a surface printing layer 35, since it is necessary to absorb the whole pressure only by the compressive layer 32, the thickness of the layers on the surface of the non-expansion layer 33 must be increased. In this case, the adhesion between the printing plate and the surface of the blanket 30 is improved and the solid inking property is improved. On the other hand, dot gain, slur and double do not arise at low-speed printing, however, the shear deformation arises in the circumferential direction of the blanket at high-speed printing at not less than 1,100 rpm because the thickness of the surface layers is larger than that of the non-expansion layer. Therefore, slur and double arise.
On the other hand, regarding a printing blanket disclosed in Japanese Patent Laid-Open Publication No. 6-270573, a surface printing layer is porous and fine pores are provided on the surface thereof for the purpose of inhibiting bulging. However, the above fine pores are generally formed by the extraction method using a water-soluble powder material such as salt and by the foaming method using micro-balloons, and it is difficult to obtain ultra-fine cells (pores) having a diameter of not more than 10 .mu.m. Therefore, a lot of circular depressions having a diameter of not less than about 10 .mu.m are formed on the surface of the blanket and the surface becomes rough so that the shape of the halftone dot is deteriorated, resulting in deterioration of the printing quality.
The surface area of the blanket surface becomes enlarged as a result of providing the porous surface printing layer and so-called piling, wherein ink and paper powder are accumulated on the blanket, is liable to arise. Consequently, the inking property of ink is liable to be deteriorated. Furthermore, when the degree of piling becomes severe, the number of washings of the blanket increases and the productivity of printing is deteriorated and, at the same time, paper loss at the time of washing increases. Accordingly, while there can be obtained such an advantage that the productivity of printing is enhanced by performing high-speed printing using a seamless blanket, there arises a problem that the productivity is deteriorated by an increase in number of washings and that undesirably high printing cost.
With the increase of the number of washings, wear of the blanket surface caused by a paper or a nonwoven fabric in blanket washing device and also scratches on the blanket surface caused by trouble over paper winding are increased. Accordingly, there arises another problem that the durability of the blanket becomes inferior.